Ligand reaction profile

Figure 12 shows the reaction profile for the hydrosilylation process involving the most stable fi3-sily 1-ally 1 complex, 10a-anti, calculated with model B. Examination of the reaction profile suggests that the rate determining step of the catalytic cycle is the reductive elimination. More specifically, the transfer of the silyl moiety to the (J-carbon of the styrene. Since recoordination of the pyrazole ligand occurs in this step, it is possible that enhancement of this ligands ability to recombined with the Pd center may lead to improved activities. 

Fig. 19 Polymerization reaction of oxetane and CO2 catalyzed by (salen)CrCl and two equivalents of n-Bu4NBr at 70°C (salen ligand contains a cyclohexylene backbone for the diimine and t-butyl substituents in the 3,5-positions of the phenolates). (a) Three-dimensional infrared traces of the closely overlapped VCO2 bonds of TMC and poly(TMC). (b) Reaction profile as a function of time, where only a select number of composite infrared bands were deconvoluted...

Fig. 1. Profile of the C-C linkage fraction (mol%) (y =J15]lll5 + 16 ) of the reaction products of the system 73/P-ligands/CO (0.5 1.0 5) 18 ligands (reaction conditions see Scheme 3.3-5). Variance of fit sj j 3.6, of measured data = 4 (estimated)...

Detailed electronic structure studies, including insights gained from ligand field theory (46), can be especially useful in interpreting the reaction profiles and understanding the reactivity and selectivity of these systems. The exploration of two-state reactivity and the value of detailed electronic structure analysis are illustrated by our studies of the H atom abstraction step catalyzed by TauD (23,47), which are presented here for each spin-state surface the hypothetical septet, the quintet, and the triplet. 

For those systems with AG° ss 0, it follows that AH0 % TAS°. Conversion from the low-spin to the high-spin state involves lengthening and weakening the metal-ligand bonds. This results in a high-spin state with higher enthalpy and also greater entropy. Such a reaction profile is shown in Fig. 5 for the Fe(HB(pz)3)2 complex. 

In 1994, asymmetric cydopropanation (ACP) with ruthenium catalysts was first reported by Nishiyama and coworkers [ 19,20] by adoption of their chiral bis(oxazolinyl)pyridine (Pybox) ligands. The reaction profiles of Ru Pybox catalysts reveal extremely high trans selectivity with high enantioselectivity (or di-astereoselectivity) of cyclopropane products at the relatively low reaction temperatures (around 20-50 °C) so far reported for ruthenium catalysts. After 1997,... 

The first consequence of relating these structures to possible intermediates for nucleophilic ligand replacements at square-planar complexes is that the simple reaction profile of Fig. 2 is inadequate. A... 

If we apply the same type of arguments to complexes of platinum and palladium, in which distinct bond-making and bond-breaking maxima apply and square-pyramidal structures are quite common, a reaction profile like Fig. 6 may well be appropriate. The trigonal-bipyramidal structure is certainly not always a transition state, however, particularly when TT-accepting ligands can adopt equatorial sites, so some reactions might well conform to the complicated profile shown in Fig. 

Fig, 5. Possible general reaction profile for gold(III) ligand replacements, dominated by a trigonal-bipyramidal transition state but allowing for square-pyramidal intermediates. 

Fig. 6. Reaction profile likely to fit most palladium(II) and platinum(II) ligand replacements.

Continuing this line of argument, we find that the predominance of trigonal-bipyramidal complexes of rhodium(I) and iridium(I) could well mean that Fig. 2, the traditional reaction profile, applies to the little-studied ligand-exchange reactions of 4-coordinate complexes of these elements. However, such conclusions, unsupported by other evidence, must remain tentative,... 

The operation of conjugate base pathways, known for many years with Au(III) compounds, is now recognized as operating at a number of Pd(II) and Pt(II) complexes. These also appear to proceed by A or la mechanisms and probably have similar reaction profiles to the normal nucleophilic ligand replacements, though there are yet too few examples to be certain. 

Figure 11.16. A simplified reaction profile for electron exchange in a symmetrical reaction. On the left of the graph, the nuclear coordinates correspond to Fe(II) and Fe(III) on the right, the ligands and solvent molecules have adjusted locations and the nuclear coordinates correspond to Fe(III) and Fe(II), where the denotes the isotope label. (Adapted from Shriver et al., 1990.)...

The dissociative pathway was chosen for theoretical investigations since it fits experimental data sufficiently [14, 15] and several test calculations disfavor the associative counterpart [7], Reaction 6 —> 7 was studied in detail on different levels of theory [7, 13]. The B3LYP/DZVP total reaction energy of hydroformylation including zero-point correction is -33.2 kcal mol , which agrees well with the experimental mean value of -28 kcal mol" per double bond [16]. Figure 10 contains the total reaction profiles of Rh- and Ir-catalyzed hydroformylation [7, 17]. Because of their low barriers [18], transition states of ligand associations/ dissociations are omitted. 

The reaction of complex (S,S)-67 or S,S)-77 prepared with excess base in benzene leads to the bis-ene-amido complexes and (Figure 10). These are active for the ATH of acetophenone without the addition of extra base. However, there is still an induction period in the catalytic reaction profile, which is associated with the reduction of one of the double bonds in the ligand backbone to produce the active catalyst, an amido(ene-amido) complex 81 (see below... 

As shown in Fig. 9, a similar experiment starting with the salen ligand and combining metal insertion and catalyst activation led to the same completion time as the previous experiment. This result demonstrates that the metal insertion step does not limit the overall rate of reaction. The similarities in reaction profile using either pure oxygen (non-scalable) or air at different stirrer speeds confirm the important role of the degree of mixing and gas-liquid dispersion in the activation reaction. Since the overall rate of the complexation and activation sequence is... 

A study on the reaction profile of the asymmetric aldol reaction catalyzed by Pr(OTf)3 with 4 revealed that his crown ether-type chiral ligand did not significantly reduce the activity of the metal triflates. This retention of the activity even in the presence of the crown ether containing oxygen and nitrogen atoms in a key to realize the asymmetric induction in this asymmetric aldol reaction in aqueous me-... 

If reactants and products are isometric, the reaction is symmetric and the reaction energy is zero. In this simple case, the transition state is symmetric and so is the reaction profile [40]. A well known example is electron self-exchange [41], which can occur whenever a chemical species exists in two or more oxidation states. Hexacoor-dinate metal complexes in oxidation states 2+ and 3+ provide an example (Figure 5.11). Initially, the metal-ligand distance di in MLg is long and the corresponding force constant fi is relatively low, whereas the distance d in is... 

REACTION PROFILES AND KINETICS FOR COORDINATION AND OXIDATIVE ADDITION OF DlHYDROGEN AND OTHER LIGANDS... 

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